OBJECTIVE: To assess whether 17 alpha-hydroxyprogesterone caproate reduces the rate of preterm birth in women carrying triplets.METHODS: We performed this randomized, double-blinded, placebo-controlled trial in 14 centers. Healthy women with triplets were randomly assigned to weekly intramuscular injections of either 250 mg of 17 alpha-hydroxyprogesterone caproate or matching placebo, starting at 16-20 weeks and ending at delivery or 35 weeks of gestation. The primary study outcome was delivery or fetal loss before 35 weeks.RESULTS: One hundred thirty-four women were assigned, 71 to 17 alpha-hydroxyprogesterone caproate and 63 to placebo; none were lost to follow-up. Baseline demographic data were similar in the two groups. The proportion of women experiencing the primary outcome (a composite of delivery or fetal loss before 35 0/7 weeks) was similar in the two treatment groups: 83% of pregnancies in the 17 alpha-hydroxyprogesterone caproate group and 84% in the placebo group, relative risk 1.0, 95% confidence interval 0.9-1.1. The lack of benefit of 17 alpha-hydroxyprogesterone caproate was evident regardless of the conception method or whether a gestational age cutoff for delivery was set at 32 or 28 weeks.CONCLUSION: Treatment with 17 alpha-hydroxyprogesterone caproate did not reduce the rate of preterm birth in women with triplet gestations.CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov, www.clinicaltrials.gov, NCT00099164LEVEL OF EVIDENCE: I.